Add Tiktokenizer link in "How to count tokens" (#604)

This adds a link to Tiktokenizer webapp as another tool, in
addition to the OpenAI Tokenizer.

examples/How_to_count_tokens_with_tiktoken.ipynb

@@ -54,7 +54,7 @@
  "\n",
  "## How strings are typically tokenized\n",
  "\n",
- "In English, tokens commonly range in length from one character to one word (e.g., `\"t\"` or `\" great\"`), though in some languages tokens can be shorter than one character or longer than one word. Spaces are usually grouped with the starts of words (e.g., `\" is\"` instead of `\"is \"` or `\" \"`+`\"is\"`). You can quickly check how a string is tokenized at the [OpenAI Tokenizer](https://beta.openai.com/tokenizer)."
+ "In English, tokens commonly range in length from one character to one word (e.g., `\"t\"` or `\" great\"`), though in some languages tokens can be shorter than one character or longer than one word. Spaces are usually grouped with the starts of words (e.g., `\" is\"` instead of `\"is \"` or `\" \"`+`\"is\"`). You can quickly check how a string is tokenized at the [OpenAI Tokenizer](https://beta.openai.com/tokenizer), or the third-party [Tiktokenizer](https://tiktokenizer.vercel.app/) webapp."
  ]
  },
  {